Over the past twenty years, many attempts have been made and described in both patent and scientific literature to successfully extract zinc in an efficient and cost-effective manner from geothermal brines, which contain various mixtures of metals and minerals. Among prior art methods was a process first performed approximately twenty years ago involving the addition of lime to precipitate metals from the brine and subsequently separating the metals. Selective sulfide precipitation was also attempted [R. W. Bartlett et al, "Sulfide Precipitation of Heavy Metals from High Salinity Geothermal Brine," Geothermal Resource Council, Transactions, 3:39-42 (1979)]. A variety of more sophisticated processes were developed, e.g., electrowinning, which involves the water stripping of a R.sub.2 NH.sub.2.sup.+ extractant followed by reextraction using di-2-ethylhexyl-phosphoric acid (D-2-EHPA) with a catholyte strip, resulting in the formation of zinc sulfate [E. D. Nogueira et al, Engineering and Mining Journal, 180(10):92-94 (1979)].
As one example of reportedly unsuccessful prior art, Byeseda, U.S. Pat. No. 4,624,704 refers to a method for selectively recovering zinc from metal-containing brines, which employs the steps of contacting brine with an organic agent to form a zinc amine, and transferring a large amount of the zinc to the organic phase, where it is then contacted with an aqueous strippant solution. While the patent reports that the zinc and zinc amine complex are transferred to an aqueous zinc chloride solution, from which zinc may be recovered by electrowinning, the present inventors have tried this method and found it produces only very low concentrations of zinc in the aqueous phase and long contact and settling times of 5 and 15 minutes, respectively. The concentration and time restraints limit the volume of brines which can be handled by the conventional extracting equipment and thus, the amount of zinc which can be produced. The low zinc concentration and Cl.sup.- accumulation in the strippant, which precludes heavy recycle, renders a technically-feasible process incapable of practical use. This method is not only not used anywhere in the world to remove zinc from brine, but the assignee of the patent has completely withdrawn from the applicable business.
Another zinc extraction process was disclosed in A. Maimoni, Geothermics. 11(4):239-258 (1982) involving a cementation process using iron as a reducing agent for cementation. In the practice of this process, a silica sludge accumulates in the equipment and requires periodic removal. It should further be noted that this process was never commercially developed.
Other prior art methods for removing zinc from brines includes, among others, U. S. Pat. No. 3,923,976; E. D. Nogueira et al, Complex Sulphide Ores, Pap. Conf., p. 227-233 (1980); E. D. Nogueria et al, Chemistry and Industry, 2:63-67 (1980); F. L. Moor et al, Plat. Surf. Finish., (August 1976); C. MacDonald, "Solvent Extraction Studies Using High Molecular Weight Amines Progress Report", Texas Southern University, Houston (July 1975); and C. MacDonald, "Removal of Toxic Metals from Metal Finishing Waste Water By Solvent Extraction", Texas Southern University, Houston U7816 (February 1978). Prior to the present invention, a vast resource of this mineral has been untapped because none of the above-referenced publications are capable of providing a successful process for extracting zinc from brines, which process is able to overcome both scientific and economic obstacles [see, e.g., E. D. Nogueira et al, "Design Features and Operating Experience of The Quimigal Zincex Plant," Chloride Electrometall., Proc. Symp.. 59-76 (1982)].
There thus remains a need in the art for a method for extracting zinc from geothermal brines which is both technically and practically feasible, and which permits zinc to be extracted efficiently and quickly from available sources.